Sorting of membrane proteins to their correct intracellular localization is essential for normal functioning of a cell. In Polycystic Kidney Diseases, a group of disorders characterized by the progressive formation of renal cysts, several membrane proteins are missorted in renal epithelial cells. One of these proteins, the epidermal growth factor receptor (EGFR), which is normally restricted to the basolateral surface, is also found at the pical surface in cystic epithelial cells. Since the cystic fluid contains EGF, a potent mitogen for renal cell-proliferation, the presence of EGFR at the apical membrane could trigger an autocrine- paracrine cycle. Thus, apical missorting of EGFR will result in epithelial cell-proliferation which is essential for cyst formation and enlargement. We have been studying the process of epithelial protein sorting at the molecular level with an emphasis on dissecting the basolateral sorting machinery and identifying the sorter proteins responsible for transport to the basolateral surface. The overall aim of this proposal is to understand why EGFR is missorted in PKD renal cells. To address this problem at the molecular level, we will first precisely delineate the basolateral sorting signal for EGFR which has already been shown to reside within a membrane proximal 71 amino acid segment of its cytoplasmic tail. Our next goal will be to identify, isolate and characterize the proteins that associate with this sorting signal in normal renal cells. We will then determine which of these proteins function as a basolateral sorter by use of a permeabilized cell system which we have developed. Finally, having identified the basolateral sorter and its role in the sorting of EGFR in normal renal cells, we will examine the interaction of EGFR with the sorter protein in cystic renal cells. A better understanding of how EGFR is sorted in normal cells and why this process fails in cystic cells should aid in designing practical strategies to limit cyst formation and enlargement in human cystic disease states.